Spray and Forget at Home Depot will kill it and clean it.
Pressure washer with Oxyclean in the tank. Many places will rent them, I have my own to get the green stuff of my siding.
Get yourself a hose attachment that has the arm power washer on the end of it and that should clean it off
If it is very close to your window frame, hire a home inspector. It may be caused by improperly installed window. If so mold may be growing on the inside too. Insurance issue then. We have had so many here in SC.
Try spritzing it with water/bleach and then scrubbing down with a stiff bristled broom if you don't have a pressure washer. Be cautious about any plantings at the base if using bleach!
Hi Ursula: Is this outside? If so, check this site and it should help. Read it very carefully as it talks about not painting:)